In material and product development, there is a need to understand material behavior before integration thereof into a final product. Therefore, there has been a need to characterize the mechanical properties of materials. For example, materials under a prolonged cyclic loading can undergo catastrophic failure due to initiation of cracks and their rapid growth. Consequently, emphasis has been placed on understanding the behavior of materials during cyclic loading; and especially during cyclic loading which simulates how that material will actually be used.
Most conventional apparatus, however, cannot measure crack formation and propagation during cyclic loading of a material. Therefore, testing of a material using a typical cyclic loading regime creates many problems. For example, monitoring surface crack formation and propagation comprises capturing an image, usually by still photography, and measuring the crack's dimensions from that image. Because the movement of the specimen during cyclic loading does not allow for clear photographic images, the loading regime must be stopped, periodically, for photographic work. Stopping the loading regime is time consuming and it allows the material to relax which is outside of the intended loading regime. Consequently, this conventional technique can lead to anomalous results.
Another method of monitoring crack growth, which has problems similar to the still photography, comprises surface replication coupled with the use of a Transmission Electron Microscope (TEM). Here the loading regime is periodically stopped to make an acetate replicate of a surface of the specimen. The replicate is then viewed under a TEM to observe crack formation and growth. This method not only comprises an undesirable relaxation period, it is very time consuming and expensive.
In addition to long down times, high cost, and alterations of the anticipated loading regime, the analysis of crack growth typically proceeds by manual calculations requiring physical measurements of the photos. The measurements are performed by placing a reference mark of predetermined length on the specimen prior to testing such that the crack lengths can be measured with respect to the reference mark. The changes in crack size can be calculated directly from the measurements of the crack, or the total crack size is determined by comparing the length of the crack to the reference mark. Since this technique includes several points of human intervention, there is a potential for error. This limitation of the monitoring apparatus can lead to an increased error in the characterization of a material, high cost, and long test periods.
Another drawback of conventional apparatus is a lack of automatic shut off based upon crack size. Because a specimen can undergo thousands of load cycles during a test, the point of crack initiation is often not recorded and the specimen is cycled until failure. Consequently, some of the early stage characteristics of a surface crack are destroyed, rendering that test specimen less valuable since the shape and pattern of a crack initiation and growth are needed for subsequent crack growth modeling.
Therefore, what is needed in the industry is an apparatus capable of accurately measuring crack growth on a specimen during the application of a load, determining a crack's dimensions in real time, and terminating the loading when the crack reaches a predetermined dimension.